Method for producing rare earth based alloy powder and method for producing rare earth based sintered magnet

ABSTRACT

An inventive method of making a rare-earth alloy powder is used to produce a rare-earth sintered magnet, whose main phase has a composition R 2 T 14 A (where R is one of the rare-earth elements including Y; T is Fe with or without a non-Fe transition metal; and A is boron with or without carbon). The method includes the steps of: preparing a first rare-earth rapidly solidified alloy, having a columnar texture with an average dendritic width within a first range, by subjecting a melt of a first rare-earth alloy with a first composition to a rapid cooling process; preparing a second rare-earth rapidly solidified alloy, having a columnar texture with an average dendritic width smaller than that of the first rare-earth rapidly solidified alloy and falling within a second range, by subjecting a melt of a second rare-earth alloy with a second composition to the rapid cooling process; making a first rare-earth alloy powder by pulverizing the first solidified alloy; making a second rare-earth alloy powder by pulverizing the second solidified alloy; and making a powder blend including the first and second rare-earth alloy powders.

TECHNICAL FIELD

The present invention relates to a method for producing a rare-earthsintered magnet (more particularly, an R—Fe—B based sintered magnet) andalso relates to a method of making a rare-earth alloy powder for use toproduce such a rare-earth sintered magnet.

BACKGROUND ART

A rare-earth alloy sintered magnet (permanent magnet) is normallyproduced by compacting a powder of a rare-earth alloy, sintering theresultant powder compact and then subjecting the sintered body to anaging treatment if necessary. Permanent magnets currently usedextensively in various applications include rare-earth-cobalt based(typically samarium-cobalt based) magnets and rare-earth-iron-boronbased (typically neodymium-iron-boron based) magnets. Among otherthings, the rare-earth-iron-boron based magnets (which will be referredto herein as “R—Fe—B based magnets”, where R is one of the rare-earthelements including Y, Fe is iron, and B is boron) are used more and moreoften in various electronic appliances. This is because an R—Fe—B basedmagnet exhibits a maximum energy product, which is higher than any ofvarious other types of magnets, and yet is relatively inexpensive.

An R—Fe—B based sintered magnet includes a main phase consistingessentially of a tetragonal R₂Fe₁₄B compound (which will be sometimesreferred to herein as an “R₂Fe₁₄B type crystal layer”), an R-rich phaseincluding Nd, for example, and a B-rich phase. In the R—Fe—B basedsintered magnet, a portion of Fe may be replaced with a transition metalsuch as Co or Ni and a portion of B may be replaced with C. An R—Fe—Bbased sintered magnet, to which the present invention is applicableeffectively, is described in U.S. Pat. Nos. 4,770,723 and 4,792,368, forexample, the entire contents of which are hereby incorporated byreference.

In the prior art, an R—Fe—B based alloy has been prepared as a materialfor such a magnet by an ingot casting process. In an ingot castingprocess, normally, rare-earth metal, electrolytic iron and ferroboronalloy as respective start materials are melted by an induction heatingprocess, and then the melt obtained in this manner is cooled relativelyslowly in a casting mold, thereby preparing a solid alloy (i.e., alloyingot). A method for obtaining a solid alloy by a Ca reduction process(which is also called a “reduction diffusion process”) is also known.

Recently, a rapid cooling process (which is also called a“melt-quenching process”) such as a strip casting process or acentrifugal casting process has attracted much attention in the art. Ina rapid cooling process, a molten alloy is brought into contact with,and relatively rapidly cooled by, a single chill roller, a twin chillroller, a rotating disk or the inner surface of a rotating cylindricalcasting mold, thereby making a solidified alloy, which is thinner thanan alloy ingot, from the molten alloy.

A solid alloy obtained by a rapid cooling process will be referred toherein as a “rapidly cooled alloy (or rapidly solidified alloy)” so asto be easily distinguished from a solid alloy obtained by a conventionalingot casting process or Ca reduction process. The rapidly solidifiedalloy typically has the shape of a flake or a ribbon (thin strip).

Compared to a solid alloy made by the conventional ingot casting processor die casting process (such an alloy will be referred to herein as an“ingot alloy”), the rapidly solidified alloy has been quenched in ashorter time (i.e., at a cooling rate of 10²° C./sec to 10⁴° C./sec).Accordingly, the rapidly solidified alloy has a finer texture and asmaller crystal grain size. In addition, in the rapidly solidifiedalloy, the grain boundary thereof has a greater area and the R-richphases are dispersed broadly and thinly over the grain boundary. Thus,the rapidly solidified alloy also excels in the dispersiveness of theR-rich phases. Because the rapidly solidified alloy has theseadvantageous features, a magnet with excellent magnetic properties canbe made from the rapidly solidified alloy.

An alloy powder to be compacted is obtained by coarsely pulverizing arapidly solidified alloy in any of these forms by a hydrogenpulverization process, for example, and/or any of various mechanicalgrinding processes (e.g., using a ball mill or attritor) and finelypulverizing the resultant coarse powder (with a mean particle size of 10μm to 500 μm) by a dry pulverization process using a jet mill, forexample. The alloy powder to be compacted preferably has a mean particlesize of 1 μm to 10 μm, more preferably 1.5 μm to 7 μm, to achievesufficient magnetic properties. It should be noted that the “meanparticle size” of a powder refers herein to an FSSS particle size unlessotherwise stated.

A rapidly solidified alloy powder obtained in this manner is typicallyprocessed into compacts by a uniaxial compacting process. Due to itsmanufacturing method, the rapidly solidified alloy powder has a narrowparticle size distribution and achieves a bad fill density (i.e., cannotfill the cavity to a desired fill density), which are both problems.

Thus, to improve the fill density of the rapidly solidified alloypowder, various countermeasures have been proposed. For example,Japanese Patent Application Laid-Open Publication No. 2000-219942describes that if a rapidly solidified alloy, including 1 vol % to 30vol % of chilled texture with particle sizes of 3 μm or less, is made bya roller rapid cooling process and then pulverized to obtain a rapidlysolidified alloy powder, then the fill density can be increased and thesintering temperature can be decreased compared with conventional ones.

It should be noted that the “chilled texture” is a crystalline phase tobe formed near the surface of a chill roller during an initial stage inwhich a melt of an R—Fe—B based rare-earth alloy has just contacted withthe surface of a cooling member (e.g., the chill roller) of a rapidcooling system and started to solidify. Compared with a columnar texture(or dendrite texture) to be formed on and after that initial stage ofthe cooling and solidification process, the chilled texture has a moreisotropic (or isometric) and finer structure. That is to say, thechilled texture is produced when the melt is rapidly cooled andsolidified on the surface of the roller.

DISCLOSURE OF INVENTION

However, the chilled texture has a magnetically isotropic finestructure. Accordingly, if a powder of a rapidly solidified alloyincludes a lot of chilled texture, then the magnetic properties of theresultant sintered magnet deteriorate.

In order to overcome the problems described above, primary objects ofthe present invention are to provide a method of making a rare-earthrapidly solidified alloy powder, which includes substantially no chilledtexture but achieves a higher fill density than a conventional one, andalso provide a method for producing a rare-earth sintered magnet byusing such a powder.

A method of making a rare-earth alloy powder according to the presentinvention is used to produce a rare-earth sintered magnet, of which amain phase has a composition represented by R₂T₁₄A (where R is one ofthe rare-earth elements including Y; T is either Fe alone or a mixtureof Fe and a transition metal element other than Fe; and A is eitherboron alone or a mixture of boron and carbon). The method includes thesteps of: preparing a first rare-earth rapidly solidified alloy, whichhas a columnar texture with an average dendritic width falling within afirst range, by subjecting a melt of a first rare-earth alloy with afirst composition to a rapid cooling process; preparing a secondrare-earth rapidly solidified alloy, which has a columnar texture withan average dendritic width that is smaller than that of the firstrare-earth rapidly solidified alloy and that falls within a secondrange, by subjecting a melt of a second rare-earth alloy with a secondcomposition to the rapid cooling process; making a first rare-earthalloy powder by pulverizing the first rare-earth rapidly solidifiedalloy; making a second rare-earth alloy powder by pulverizing the secondrare-earth rapidly solidified alloy; and making a powder blend includingthe first and second rare-earth alloy powders, whereby the objectsdescribed above are achieved.

In one embodiment, the first range is equal to or greater than the meanparticle size of the first rare-earth alloy powder, and the secondrange- is less than the mean particle size of the second rare-earthalloy powder.

The first range is preferably from 3 μm through 6 μm while the secondrange is preferably from 1.5 μm through 2.5 μm.

A method of making a rare-earth alloy powder according to anotherembodiment includes the steps of: obtaining a first rare-earth alloycoarse powder by coarsely pulverizing the first rare-earth rapidlysolidified alloy; obtaining a second rare-earth alloy coarse powder bycoarsely pulverizing the second rare-earth rapidly solidified alloy;making a blended coarse powder by blending the first and secondrare-earth alloy coarse powders together; and obtaining the powder blendhaving a mean particle size of 1 μm to 10 μm by finely pulverizing theblended powder.

A method of making a rare-earth alloy powder according to anotherembodiment includes the steps of: making a first rare-earth powderhaving a mean particle size of 1 μm to 10 μm from the first rare-earthrapidly solidified alloy; making a second rare-earth powder having amean particle size of 1 μm to 10 μm from the second rare-earth rapidlysolidified alloy; and obtaining the powder blend by blending the firstand second rare-earth powders together.

The first and second rare-earth alloy powders included in the powderblend preferably have a volume percentage ratio of 95:5 through 60:40,more preferably 80:20 through 70:30.

In another embodiment, the second rare-earth rapidly solidified alloy ismade by a strip casting process.

In another embodiment, the first rare-earth rapidly solidified alloy ismade by a strip casting process.

In another embodiment, the first rare-earth rapidly solidified alloy ismade by a centrifugal casting process.

In another embodiment, the first rare-earth rapidly solidified alloyincludes 30 mass % to 32 mass % of R. In another embodiment, the secondrare-earth rapidly solidified alloy includes 33.5 mass % to 35 mass % ofR.

A rare-earth sintered magnet producing method according to the presentinvention is a method for producing a rare-earth sintered magnet, ofwhich a main phase has a composition represented by R₂T₁₄A (where R isone of the rare-earth elements including Y; T is either Fe alone or amixture of Fe and a transition metal element other than Fe; and A iseither boron alone or a mixture of boron and carbon). The methodincludes the steps of: preparing a rare-earth alloy powder by one of themethods described above; compacting a powder material, including therare-earth alloy powder, thereby obtaining a compact; and sintering thecompact, whereby the object described above are achieved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a micrograph showing a cross section of a rapidly solidifiedalloy including substantially no chilled texture.

FIG. 2 is a micrograph showing a cross section of a rapidly solidifiedalloy including a chilled texture.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of a method for producing an R—Fe—Bbased rare-earth sintered magnet according to the present invention willbe described with reference to the accompanying drawings.

In this description, the composition of the main phase of an R—Fe—Bbased sintered magnet is represented by a general formula R₂T₁₄A. Thismain phase has an R₂T₁₄A type (Nd₂Fe₁₄B type) crystal structure(tetragonal).

In this formula, R is one of the rare-earth elements (including Y), T iseither Fe alone or a mixture of Fe and at least one transition metalelement other than Fe, and A is either boron alone or a mixture of boronand carbon. It should be noted that the rare-earth element R preferablyincludes at least one light rare-earth element such as Nd or Pr andpreferably further includes at least one heavy rare-earth elementselected from the group consisting of Dy, Ho and Tb to ensure goodcoercivity. The light rare-earth element preferably accounts for 50atomic % or more of the overall rare-earth element R. Examples of thenon-Fe transition metal elements include Ti, V, Cr, Mn, Fe, Co and Ni. Tpreferably either consists essentially of Fe alone or consists mostly ofFe, a portion of which is replaced with at least one of Ni and Co.

To achieve good magnetic properties, the overall composition of thesintered magnet preferably includes 25 mass % to 40 mass % of R, 0.6mass % to 1.6 mass % of A, and T and very small amounts of additives(and inevitably contained impurities) as the balance. The very smallamounts of additives preferably include at least one element selectedfrom the group consisting of Al, Cu, Ga, Cr, Mo, V, Nb and Mn. The totalamount of those additives introduced is preferably at most 1 mass % ofthe overall magnet.

The present inventors analyzed the relationship between the powder filldensity and texture of a rapidly solidified alloy from various angles tomake the following discoveries, which formed the basis of the presentinvention.

A melt of a rare-earth alloy material having the desired compositiondescribed above is prepared and rapidly cooled and solidified to make arapidly solidified alloy. In this process, the resultant rapidlysolidified alloy may have any of various textures depending on thatcomposition and/or specific conditions of the rapid cooling process.

For example, in making a rapidly solidified alloy by a strip castingprocess (see, for example, U.S. Pat. No. 5,666,635, the entire contentsof which are hereby incorporated by reference), if the circumferentialvelocity of the chill roller is relatively high, then a structure with achilled texture such as that shown in FIG. 2 is formed. Thecross-sectional micrograph of the rapidly solidified alloy shown in FIG.2 includes about 10 vol % of chilled texture.

On the other hand, if the circumferential velocity of the roller isrelatively low, then a structure consisting essentially of a dendritetexture (i.e., columnar texture or columnar crystals) alone andincluding substantially no chilled texture is formed as shown in FIG. 1.Also, even if a number of structures each consist essentially of thedendrite texture, the dendritic widths thereof are changeable with thecircumferential velocity of the roller. Specifically, the lower thecircumferential velocity, the broader the dendritic width.

Such a difference in texture between rapidly solidified alloys alsodepends on the composition of the alloy. For example, when a number ofalloys were compared on the same rapid cooling conditions (e.g., at thesame chill roller circumferential velocity), the higher the R content ofthe alloy, the narrower the dendritic width thereof tended to be.

A number of rapidly solidified alloys with mutually different textureswere obtained in this manner. Then, each of those alloys was subjectedto pulverization, compaction and sintering process steps underpredetermined conditions, thereby making a sintered magnet. The magneticproperties of the resultant sintered magnets were evaluated and the filldensities of the alloy powders that were subjected to the compactionprocess were estimated. As a result, the present inventors discoveredthat if a plurality of alloy powders, made from rapidly solidifiedalloys with mutually different dendritic widths, were blended and used,then the fill density of the blended alloy powder increased and themagnetic properties of the resultant sintered magnet improved. This isbelieved to be because if those rapidly solidified alloys with mutuallydifferent dendritic widths are pulverized, then powders with differentparticle size distributions corresponding to the respective dendriticwidths are obtained, and therefore, the particle size distribution ofthe blended powder broadens. This would also be because the powderparticles, made from the rapidly solidified alloys with mutuallydifferent dendritic widths, have different aspect ratios. For example,by controlling the dendritic widths of the two rapidly solidifiedalloys, making up the single blended powder, such that one of the tworapidly solidified alloys has an average dendritic width equal to orgreater than the mean particle size thereof and that the other rapidlysolidified alloy has an average dendritic width less than the meanparticle size thereof, a powder, made up of two groups of particles withmutually different aspect ratios, can be obtained.

It should be noted that the dendritic width, characterizing such arapidly solidified alloy, is supposed herein to be the average of thetwo different dendritic widths (which will be referred to herein as the“average dendritic width”). The average dendritic width was obtained bycounting the number of dendrites included within a certain range (with awidth of 20 μm to 50 μm, for example) and calculating the average. Sucha method is sometimes called a “line segment method”. The number ofsamples was supposed to be at least five.

A method of making a rare-earth alloy powder according to the presentinvention includes the steps of: (a) preparing a first rare-earthrapidly solidified alloy, which has a columnar texture with an averagedendritic width falling within a first range, by subjecting a melt of afirst rare-earth alloy with a first composition to a rapid coolingprocess; (b) preparing a second rare-earth rapidly solidified alloy,which has a columnar texture with an average dendritic width that issmaller than that of the first rare-earth rapidly solidified alloy andthat falls within a second range, by subjecting a melt of a secondrare-earth alloy with a second composition to the rapid cooling process;(c) making a first rare-earth alloy powder by pulverizing the firstrare-earth rapidly solidified alloy; (d) making a second rare-earthalloy powder by pulverizing the second rare-earth rapidly solidifiedalloy; and making a powder blend including the first and secondrare-earth alloy powders.

The first range is preferably from 3 μm through 6 μm while the secondrange is preferably from 1.5 μm through 2.5 μm. The reasons are asfollows. Specifically, if the average dendritic width of the firstrare-earth alloy powder exceeded 6 μm, then the coercivity mightdecrease unfavorably. However, if the average dendritic width were lessthan 3 μm, then the fill density might decrease, which is notbeneficial, either. On the other hand, if the average dendritic width ofthe second rare-earth alloy powder exceeded 2.5 μm, then the filldensity and/or the sinterability might decrease unfavorably. However, ifthe average dendritic width were less than 1.5 μm, then it would bedifficult to produce a uniformly texture.

The average dendritic width of the first rare-earth alloy powder ispreferably defined equal to or greater than the mean particle sizethereof, but the average dendritic width of the second rare-earth alloypowder is preferably defined less than the mean particle size thereof.With these settings, the aspect ratio of particles of the firstrare-earth alloy powder should be different from that of particles ofthe second rare-earth alloy powder, and therefore, the fill density oftheir blend should improve. This is particularly effective if the meanparticle sizes of the first and second rare-earth alloy powders aresubstantially equalized with each other.

The first and second rare-earth alloy powders included in the blendedpowder preferably have a volume percentage ratio of 95:5 through 60:40,more preferably from 80:20 through 70:30. This is because if theblending ratio fell out of any of these ranges, the fill density couldnot be increased sufficiently. Optionally, not only the first and secondrare-earth alloy powders but also a third rare-earth alloy powder with adifferent average dendritic width may be blended together.

The rapidly solidified alloys with different average dendritic widthsmay be obtained by changing the rapid cooling rates, for example. When astrip casting process is adopted, the rapid cooling rate may be adjustedby changing the circumferential velocity of the chill roller, forexample. The strip casting process excels in mass productivity, which isvery beneficial. The rapidly solidified alloy with a relatively broaddendritic width may also be made by a centrifugal casting processresulting in a relatively low rapid cooling rate.

Alternatively, the rapidly solidified alloys with different averagedendritic widths may also be obtained by changing the compositions ofthe alloy materials. It is naturally possible to adjust both the alloymaterial composition and the rapid cooling rate alike. For example, whenthe rapidly solidified alloys are made by a strip casting process, thefirst rare-earth rapidly solidified alloy preferably includes 30 mass %to 32 mass % of R, while the second rare-earth rapidly solidified alloypreferably includes 33.5 mass % to 35 mass % of R. If the compositionsof the first and second rare-earth alloys fell out of these ranges, thenit would be difficult to obtain rapidly solidified alloys with thedesired dendritic widths.

The blending process step for obtaining the blend of first and secondrare-earth alloy powders, obtained from the rapidly solidified alloyswith different average dendritic widths, may be carried out at anappropriate point in time. Each of the rapidly solidified alloys istypically a flake and needs to go through a two-stage pulverizationprocess (i.e., a coarse pulverization process step and a finepulverization process step) before the alloy powder to be subjected tothe compaction process step is obtained. As to this pulverizationprocess, the rapidly solidified alloys may be blended together at anytime. That is to say, it does not matter if it is when the rapidlysolidified alloys are still flakes, after the rapidly solidified alloyflakes have been coarsely pulverized into coarse powders, or after thecoarse powders have been finely pulverized into fine powders(corresponding to the first and second rare-earth alloy powdersdescribed above).

Nevertheless, to minimize the oxidation of the alloy materials, not somuch the fine powders as the alloy flakes or coarse powders arepreferably blended together. In that case, the blending process step andthe pulverization process step may be carried out at the same time.Naturally, before the blending ratio is determined, the compositions ofthe respective rare-earth alloy materials (in the form of alloy flakes,coarse powders or fine powders) are preferably analyzed.

The alloy powder to be eventually compacted preferably has a meanparticle size of about 1 μm to about 10 μm, more preferably 1.5 μm to 7μm. To minimize the oxidation and/or improve the flowability orcompactibility, the surface of the rapidly solidified alloy powder maybe coated with a lubricant if necessary. It is preferable that thelubricant is added during the process step of finely pulverizing therapidly solidified alloy coarse powder. As the lubricant, a liquidlubricant consisting essentially of a fatty acid ester can be usedeffectively.

A compact is made by compacting the blended powder thus obtained by aknown compaction method. Then, the compact is processed by known methodsto complete a sintered magnet.

The rapidly solidified alloy powder (blended powder) may be compacted(e.g., uniaxially compacted and compacted) with a motorized press at apressure of 1.5 ton/cm² (i.e., 150 MPa) while being aligned under amagnetic field of about 1.5 T, for example. In this process step, whenthe cavity of the press machine is filled with the rapidly solidifiedalloy powder, a fill density higher than the conventional one isachieved because the rapidly solidified alloy powder of this preferredembodiment of the present invention has excellent loadability.Accordingly, a sintered body with a predetermined density can beobtained even at a relatively low temperature. That is to say, since itis possible to prevent the crystal grains from growing excessivelyduring the sintering process step, a sintered magnet with highercoercivity than a conventional one can be obtained.

Next, the resultant compact is sintered at a temperature of about 1,000°C. to about 1,100° C. for approximately one to five hours within eitheran inert gas (such as rare gas or nitrogen gas) atmosphere (preferablyat a reduced pressure) or a vacuum, for example. Subsequently, bysubjecting the resultant sintered body to an aging treatment at atemperature of about 450° C. to about 800° C. for approximately one toeight hours, an R—Fe—B based alloy sintered body can be obtained.Optionally, in order to reduce the amount of carbon included in thesintered body and thereby improve the magnetic properties, the lubricantthat covers the surface of the alloy powder may be heated and removed ifnecessary before the sintering process step. This lubricant removalprocess step may be carried out at a temperature of about 100° C. toabout 600° C. for approximately three to six hours within a reducedpressure atmosphere, although these conditions may vary with the type ofthe lubricant used.

Then, by magnetizing the resultant sintered body, a sintered magnet iscompleted. The magnetizing process step may be carried out at anarbitrary point in time after the sintering process step is over, andcould be performed after the magnet has been embedded in a motor or anyother device. The magnetizing magnetic field may have a strength of 2MA/m or more, for example.

EXAMPLES

Hereinafter, a method for producing an R—Fe—B based sintered magnetaccording to the present invention will be described by way of specificexamples. However, the present invention is in no way limited to thefollowing specific examples.

A first rare-earth alloy may have a composition including 31.3 mass % ofNd+Pr+Dy (of which 1.2 mass % to 2.0 mass % is Dy and the rest is Nd andPr), 1.0 mass % of B, 0.9 mass % of Co, 0.2 mass % of Al, 0.1 mass % ofCu, and Fe and inevitably contained impurities as the balance. The firstrare-earth alloy with this composition was melted at about 1,350° C.,and a rapidly solidified alloy (alloy flakes) was made from theresultant molten alloy by a strip casting process. By setting thecircumferential velocity of the chill roller to 60 m/min, alloy flakeswith a thickness of about 0.3 mm were obtained. When observing the crosssection of these alloy flakes with a microscope, the present inventorsconfirmed that the rapidly solidified alloy included substantially nochilled texture and consisted essentially of columnar crystals alone.The average dendritic width was about 4 μm.

On the other hand, a second rare-earth alloy may have a compositionincluding 34.5 mass % of Nd+Pr+Dy (of which 1.0 mass % to 2.0 mass % isDy and the rest is Nd and Pr), 1.0 mass % of B, 0.9 mass % of Co, 0.2mass % of Al, 0.1 mass % of Cu, and Fe and inevitably containedimpurities as the balance. The second rare-earth alloy with thiscomposition was melted at about 1,350° C., and a rapidly solidifiedalloy (alloy flakes) was made from the resultant molten alloy by a stripcasting process. By setting the circumferential velocity of the chillroller to 90 m/min, alloy flakes with a thickness of about 0.2 mm wereobtained. When observing the cross section of these alloy flakes with amicroscope, the present inventors confirmed that the rapidly solidifiedalloy included substantially no chilled texture and consistedessentially of columnar crystals alone. The average dendritic width wasabout 2 μm.

Example No. 1

In this example, the flakes of the first and second rare-earth alloysobtained as described above were coarsely pulverized separately by ahydrogen pulverization process, for example. The resultant coarsepowders were blended together with a rocking mixer. The blending ratiowas 75:25 on a volume basis.

Then, the resultant blended coarse powder was finely pulverized with ajet mill to a mean particle size of about 3 μm. Optionally, before thecoarse powders are blended together, those powders may be put into thejet mill by a predetermined amount so as to be blended together whilebeing finely pulverized. Thereafter, about 0.3 mass % of a lubricantconsisting essentially of a fatty acid ester was added thereto and mixedwith them.

The resultant blended powder was compacted and compacted (at a pressureof 1 ton/cm² and under an aligning magnetic field of 1.5 T), therebyobtaining a compact (with dimensions of 18 mm vertically, 55 mmhorizontally and 25 mm in the height (or pressing) direction). It shouldbe noted that the aligning magnetic field was applied perpendicularly tothe compacting direction. The compact thus obtained had a mass of 100 g.

Thereafter, the compact was sintered at 1,050° C. for four hours withina reduced pressure Ar atmosphere and then subjected to an agingtreatment at 500° C. for one hour. Subsequently, the sintered body wasmagnetized with a pulse magnetizer and then the magnetic properties ofthe resultant sintered magnet were evaluated with a search coil and aflux meter. The fill density was measured with a tap denser. As usedherein, the “fill density” refers to a tap density obtained with the tapdenser. The results are shown in the following Table 1.

Example No. 2

As in the first example described above, coarse powders of the first andsecond rare-earth alloys were obtained. Then, the coarse powders werefinely pulverized separately with a jet mill, thereby obtaining firstand second rare-earth alloy powders with a mean particle size of about 3μm. By blending these fine powders at a ratio of 75:25 using a rockingmixer, a blended powder was obtained. Thereafter, a sintered magnet wasobtained and the magnetic properties thereof were evaluated as in thefirst example described above.

Example No. 3

A sintered magnet was produced as in the first example described aboveexcept that the first rare-earth rapidly solidified alloy was made by acentrifugal casting process. The present inventors confirmed that thefirst rare-earth rapidly solidified alloy, made by the centrifugalcasting process, included substantially no chilled texture and consistedessentially of columnar crystals only. The average dendritic width wasabout 20 μm.

Comparative Example No. 1

The rare-earth alloy had a composition including 32.0 mass % of Nd+Pr+Dy(of which 1.0 mass % to 2.0 mass % was Dy and the rest was Nd and Pr),1.0 mass % of B, 0.9 mass % of Co, 0.2 mass % of Al, 0.1 mass % of Cu,and Fe and inevitably contained impurities as the balance. The firstrare-earth alloy with this composition was melted at about 1,350° C.,and a rapidly solidified alloy (alloy flakes) was made from theresultant molten alloy by a strip casting process. By setting thecircumferential velocity of the chill roller to 100 m/min, alloy flakeswith a thickness of about 0.3 mm were obtained. When observing the crosssection of these alloy flakes with a microscope, the present inventorsconfirmed that the rapidly solidified alloy included 10 vol % of chilledtexture. Thereafter, as in the first example described above, the alloyflakes were coarsely and then finely pulverized to obtain a compact,which was then processed into a sintered magnet.

Comparative Example No. 2

A rapidly solidified alloy (alloy flakes) was made by a strip castingprocess from a rare-earth alloy with the same composition as the firstcomparative example. By setting the circumferential velocity of thechill roller to 70 m/min, alloy flakes with a thickness of about 0.3 mmwere obtained. When observing the cross section of these alloy flakeswith a microscope, the present inventors confirmed that the rapidlysolidified alloy included substantially no chilled texture. Thereafter,as in the first example described above, the alloy flakes were coarselyand then finely pulverized to obtain a compact, which was then processedinto a sintered magnet.

Comparative Example No. 3

A rapidly solidified alloy was made by a centrifugal casting processfrom a rare-earth alloy with the same composition as the firstcomparative example. When observing the cross section of this rapidlysolidified alloy with a microscope, the present inventors confirmed thatthe rapidly solidified alloy included substantially no chilled texturebut consisted essentially of columnar crystals only. The averagedendritic width was about 25 μm. Thereafter, as in the first exampledescribed above, the rapidly solidified alloy was coarsely and thenfinely pulverized to obtain a compact, which was then processed into asintered magnet. TABLE 1 Comp. Example 1 Example 2 Example 3 Ex. 1 Comp.Ex. 2 Comp. Ex. 3 B_(r) 1.37 1.37 1.36 1.34 1.33 1.33 (T) H_(cJ) 1233.51233.5 1074.3 1193.7 1114.1 994.8 (kA/m) BH_(max) 362 362 358 358 354350 (kJ/m³) Fill 2.1 2.2 2.2 2.0 2.0 2.0 density (g/cm³) Sintering 1,0401,040 1,060 1,050 1,040 1,080 Temp. (° C.)

As can be seen from the results shown in Table 1, the rare-earth alloypowders (blended powders) of Examples Nos. 1 to 3 achieve higher filldensities than the non-blended powders of Comparative Examples Nos. 1 to3. Accordingly, even when sintered at relatively low sinteringtemperatures, the rare-earth alloy powders of Examples Nos. 1 to 3 stillachieved a desired density of 7.5 g/cm³ and high coercivity H_(cJ).

Example No. 3 that used the first rare-earth rapidly solidified alloy(with an average dendritic width of about 20 μm) made by a centrifugalcasting process did not exhibit as good magnetic properties as ExamplesNos. 1 and 2 that used the first rare-earth rapidly solidified alloy(with an average dendritic width of about 4 μm) made by a strip castingprocess. Thus, it can be seen that the strip casting process is apreferred method for making the rapidly solidified alloy.

Next, the results of experiments the present inventors carried out todefine a preferred range of average dendritic widths will be described.

With alloys having the same compositions as those described for thespecific examples of the present invention used as the first and secondrare-earth alloys but with the conditions of the strip casting processchanged, first and second rare-earth rapidly solidified alloys withmutually different dendritic widths were obtained. The average dendriticwidths of respective samples are shown in the following Table 2. Afterthe first and second rare-earth rapidly solidified alloys were obtainedin this manner, sintered magnets were produced as in the second exampledescribed above except that the sintering temperatures were set as shownin the following Table 3. The present inventors evaluated the magneticproperties of the resultant sintered magnets. The results are also shownin the following Table 3. TABLE 2 Average dendritic width Averagedendritic width of 1^(st) rare-earth of 2^(nd) rare-earth rapidly SampleNo. rapidly solidified alloy solidified alloy 1 6 μm 1.5 μm 2 6 μm 2.5μm 3 3 μm 1.5 μm 4 8 μm   2 μm

TABLE 3 Sample 1 Sample 2 Sample 3 Sample 4 B_(r) (T) 1.38 1.38 1.371.38 H_(cJ) (kA/m) 1215.5 1215.3 1223.5 1154.0 BH_(max) (kJ/m³) 366 366362 366 Fill density 2.2 2.2 2.2 2.2 (g/cm³) Sintering Temp. 1,040 1,0401,040 1,050 (° C.)

As can be seen from Table 3, Sample No. 4, of which the first rare-earthrapidly solidified alloy had an average dendritic width of 8 μm, hadlower coercivity H_(cJ) than any other sample. Accordingly, to achievesufficient coercivity, the first rare-earth rapidly solidified alloypreferably has an average dendritic width of 6 μm or less. It should benoted that the greater the average dendritic width of the firstrare-earth rapidly solidified alloy, the higher the remanence B_(r)tends to be and the lower the coercivity H_(cJ) tends to be.

As long as the average dendritic width of the second rare-earth rapidlysolidified alloy falls within the range of 1.5 μm to 2.5 μm, there issubstantially no sensible difference in magnetic properties. Naturally,if the average dendritic width of the first rare-earth alloy powder wereless than 3 μm and if that of the second rare-earth alloy powderexceeded 2.5 μm, then the fill density, which should be increased byblending the two types of rare-earth alloy powders together, would notincrease anymore. Also, as a result of various experiments, the presentinventors discovered that it was difficult to obtain a rare-earthrapidly solidified alloy with an average dendritic width of less than1.5 μm. Thus, the minimum average dendritic width would be 1.5 μm.

Next, results of experiments, which were carried out to find the bestrange of the blending ratio (volume ratio) by using the same first andsecond rare-earth alloy powders as those of the second example, will bedescribed. The following Table 4 shows the volume ratios of the firstand second rare-earth alloy powders and the fill densities (tapdensities) that were measured with a tap denser: TABLE 4 Sam- Sam- pleple Sample Sample Sample Sample 5 6 7 8 9 10 Volume ratio 95:5 80:2070:30 60:40 50:50 30:70 (FIRST:SECOND) Fill density 2.1 2.2 2.2 2.1 1.91.8 (g/cm³)where the volume ratio is the ratio of the volume of the firstrare-earth alloy powder to that of the second rare-earth alloy powder.

As can be seen from the results shown in Table 4, the volume ratio ofthe first rare-earth alloy powder to the second rare-earth alloy powderpreferably falls within the range of 95:5 to 60:40 (in particular, 80:20to 70:30). It is not quite clear why the fill density is improved byadopting such a blending ratio. But such a volume ratio is believed tobe effective in closing the gap, created by the first rare-earth alloypowder, with the second rare-earth alloy powder.

INDUSTRIAL APPLICABILITY

The present invention provides a method of making a rare-earth rapidlysolidified alloy powder, which includes substantially no chilled texturebut achieves a higher fill density than a conventional one, and alsoprovides a method for producing a rare-earth sintered magnet by usingsuch a powder.

1. A method of making a rare-earth alloy powder for use to produce arare-earth sintered magnet, of which a main phase has a compositionrepresented by R₂T₁₄A (where R is one of the rare-earth elementsincluding Y; T is either Fe alone or a mixture of Fe and a transitionmetal element other than Fe; and A is either boron alone or a mixture ofboron and carbon), the method comprising the steps of: preparing a firstrare-earth rapidly solidified alloy, which has a columnar texture withan average dendritic width falling within a first range, by subjecting amelt of a first rare-earth alloy with a first composition to a rapidcooling process; preparing a second rare-earth rapidly solidified alloy,which has a columnar texture with an average dendritic width that issmaller than that of the first rare-earth rapidly solidified alloy andthat falls within a second range, by subjecting a melt of a secondrare-earth alloy with a second composition to the rapid cooling process;making a first rare-earth alloy powder by pulverizing the firstrare-earth rapidly solidified alloy; making a second rare-earth alloypowder by pulverizing the second rare-earth rapidly solidified alloy;and making a powder blend including the first and second rare-earthalloy powders.
 2. The method of claim 1, wherein the first range isequal to or greater than the mean particle size of the first rare-earthalloy powder, and the second range is less than the mean particle sizeof the second rare-earth alloy powder.
 3. The method of claim 1, whereinthe first range is from 3 μm through 6 μm.
 4. The method of one ofclaims 1, wherein the second range is from 1.5 μm through 2.5 μm.
 5. Themethod of claim 1, comprising the steps of: obtaining a first rare-earthalloy coarse powder by coarsely pulverizing the first rare-earth rapidlysolidified alloy, obtaining a second rare-earth alloy coarse powder bycoarsely pulverizing the second rare-earth rapidly solidified alloy;making a blended coarse powder by blending the first and secondrare-earth alloy coarse powders together; and obtaining the powder blendhaving a mean particle size of 1 μm to 10 μm by finely pulverizing theblended powder.
 6. The method of claim 1, comprising the steps of:making a first rare-earth powder having a mean particle size of 1 μm to10 μm from the first rare-earth rapidly solidified alloy; making asecond rare-earth powder having a mean particle size of 1 μm to 10 μmfrom the second rare-earth rapidly solidified alloy; and obtaining thepowder blend by blending the first and second rare-earth powderstogether.
 7. The method of claim 1, wherein the first and secondrare-earth alloy powders included in the powder blend have a volumepercentage ratio of 95:5 through 60:40.
 8. The method of claim 1,wherein the second rare-earth rapidly solidified alloy is made by astrip casting process.
 9. The method of claim 1, wherein the firstrare-earth rapidly solidified alloy is made by a strip casting process.10. The method of claim 1, wherein the first rare-earth rapidlysolidified alloy is made by a centrifugal casting process.
 11. Themethod of claim 1, wherein the first rare-earth rapidly solidified alloyincludes 30 mass % to 32 mass % of R.
 12. The method of claim 1, whereinthe second rare-earth rapidly solidified alloy includes 33.5 mass % to35 mass % of R.
 13. A method for producing a rare-earth sintered magnet,of which a main phase has a composition represented by R₂T₁₄A (where Ris one of the rare-earth elements including Y; T is either Fe alone or amixture of Fe and a transition metal element other than Fe; and A iseither boron alone or a mixture of boron and carbon), the methodcomprising the steps of: preparing a rare-earth alloy powder by themethod of claim 1; compacting a powder material, including therare-earth alloy powder, thereby obtaining a compact; and sintering thecompact.